taskbar: Fix multiple groups when WindowClass changes
taskbar: Fix multiple groups when WindowClass changes

We need to propagate windowChanged to each group if WindowClass changes

see lxde/lxqt#586

stefonarch/lxqt-panel,stefonarch/lxqt-panel,pmattern/lxqt-panel,ThomasVie/lxqt-panel-de,cferr/lxqt-panel,lxde/lxqt-panel,Ilya87/lxqt-panel,stefonarch/lxqt-panel,pmattern/lxqt-panel,DrTobe/lxqt-panel,DrTobe/lxqt-panel,cferr/lxqt-panel,ThomasVie/lxqt-panel-de,Ilya87/lxqt-panel,lxde/lxqt-panel